Corrosion-resistant alloys



Nov. 13, 1962 M. STERN CORROSION-RESISTANT ALLOYS Filed June 18, 1959 2Sheets-Sheet 2 l I U! I 40,000 Ol/. H2504 I os'/. H2504 I 43-40244 s04,000 2 4 4| H :4,000 400 407. BOILING H2504 a H M 2 0 U I 400 g s 4 I EI I o E o 5/. BOILING H2804 I, I E n: i m 2 g i 4 o 40 77 8 0 g F: g 8 Q0 2 O 8 E 47. BOILING H2504 I l J LO 4 E I 4 4 4 444; 0 0.4 0.2 0.3 0.40.5 0?! .20

PER CENT PLATINUM INVENTOR. MILTON STERN BY fimfi z) A T TORNEV UnitedStates Patent Ofitice 3,063,835 Patented Nov. 13, 1962 3,063,835CORROSION-RESISTANT ALLOYS Milton Stern, Tonawanda, N.Y., assignor toUnion Carbide Corporation, a corporation of New York Filed June 18,1959, Ser. No. 821,134 Claims. (Cl. 75175.5)

This invention relates to corrosion-resistant alloys and, particularly,to corrosion resistant alloys in which titanium or chromium is thepredominant metal.

Titanium and many of its alloys, as well as chromium and many of itsalloys are noted for excellent resistance to oxidizing corrosive media.In non-oxidizing corrosive media, such as hydrochloric and sulfuric acidsolutions, these metals and alloys exhibit little or no resistance. Itwould be most desirable to have available alloys capable of withstandingcorrosive environments ranging from strongly oxidizing to weaklyoxidizing or non-oxidizing. An alloy of this type would have the widestutility. For example, such an alloy would be invaluable in chemicalplant equipment.

Hydrochloric and sulfuric acids are prime examples of non-oxidizingcorrosive media. These acids enter into many important commercialprocesses. They require storing and handling equipment which combine thedesirable properties cited above. If a titanium alloy, for example, wereto possess resistance to non-oxidizing acid solutions, in addition toits natural endowments of lightness, strength, and resistance tooxidizing acid solutions, it would be extremely useful in suchapplications.

There have been attempts to design titanium alloys specifically toresist attack by a strong non-oxidizing acid. Most typical among theseare compositions of 30 to 40 percent molybdenum, balance substantiallyall titanium. The corrosion resistance is derived in great measure fromthe molybdenum which is itself resistant to these acids, but theeitectiveness of the molybdenum does not become especially significantuntil amounts up to 20 percent are added. One disadvantage of this alloyis that the tendency toward brittleness increases along with molybdenumcontent so that by the time 40 percent is reached, the alloy has becomeexcessively brittle and difiicult to fabricate. Moreover, as resistanceto non-oxidizing acids is gained, the resistance to oxidizing acids islost.

One widely used method for preventing corrosion is to add passivatinginhibitors to the environment which, it is believed, operate byproducing local action current to anodically polarize a metal into thepassive potential region. It is not always desirable, however, to alterthe composition of a solution to prevent corrosion of the equipment inwhich it is contained, particularly when the composition of the solutionis critical or specific to some process. Such a means of preventingcorrosion is limited to the particular case where the composition of thecontained solution is not critical.

According to scientific theory, corosion resistance can be achieved innon-oxidizing environments by artificially anodically polarizing thebase metal by applying an external current (which might be termed anodicprotection as opposed to cathodic protection). This method, for obviousreasons, would be extremely difiicult to accomplish in chemical plantequipment.

It is, therefore, the primary object of this invention to provide alloysof titanium and alloys of chromium which are resistant to the attack ofoxidizing and non-oxidizing corrosive media.

It is also an object of this invention to provide a method forincreasing the resistance to non-oxidizing corrosive media of the metalstitianum, chromium, and their alloys.

Other aims and advantages of this invention will be apparent from thefollowing description and the appended claims.

In accordance with these objects a corrosion resistant alloy is providedconsisting essentially of from 0.005 to 5 percent by weight in theaggregate of at least one metal selected from the group consisting ofruthenium, rhodium, palladium, osmium, iridium, platinum, gold, rhenium,and alloys thereof, and the balance a metal selected from the groupconsisting of titanium, chromium, and alloys thereof wherein saidselected metal is present in predominant amounts.

It is believed that the important factor in obtaining the desiredimprovement, and the principle upon which the in-- vention operates,lies in the production of suflicient local action current to anodicallypolarize the alloy into the passive region. High local action currentsare created by high potential differences between the electrodes of thegalvanic couple and by shallow polarization curve slopes. The currentrequired to change the potential of either cathode or anode a givenamount by polarization is proportional to the cathode and anode areaswhich, in turn, are determined by the noble metal content of the alloy.

Specific embodiments of alloys of the invention have been prepared andtested. Dramatic improvements in corrosion rates were obtained and areset forth in the following table in comparison with pure titanium andvarious titanium alloys. In these tests, the samples were degreased,pickled, dried, and weighed. They were then immersed in boiling acid ofcomposition and concentration as set forth, for one period of 24 hoursduration, removed, washed, and weighed again. The corrosion rate wascalculated and reported as mils penetration per year.

Table I indicates the results of noble metal additions to substantiallypure titanium. Table II provides the same information as to noble metaladditions to binary and tertiary titanium base alloys. The percentagesgiven in each case are percentages by weight of the total alloy.

TABLE I Effect of Noble Metal Additions ion Corrosion Rate of TitaniumCorrosion rate (mils/year) Composition, percent noble metal Boiling H2804 Boiling H01 Ti 455 3 2, 952 6 241. 8 4, 479. 7 Ti+0.01 Pt- 9. 0 1,411. 8 Ti+0.37 Pt- 1. 3 39. 0 1. 0 71. 2 Ti+2.00 Pt 1. 0 36. 0 Ti+5.00Pt 89. 6 Ti+0.005 Pd 56.1 3,125. 4 Ti+0.44 Pd--- 1. 3 0. 6 67. 3 Ti-HL48AIL--- 3. 2 206. 4 8. 7 145. 5 Ti+0.60 Ir Nil 44. 8 2. 6 87. 6 Ti+0.48Os. 1. 0 81. 5 2. 6 207. 7 ii+0.36 Re- 8.7 29.0 Ti+0.5 Rh.. 2.6 48. 0 1.6 55.1 TH-0.5 Run"- 1.6 48.0 2. 3 112. 7

7 sufficient to meet the particular I 3 TABLE n Effect of Noble MetalAdditions on Corrosion Rate of Titanium Base Alloys Corrosion rate(mils/year) Composition, percent additive BoilingHzs O4 Boiling H01 .5Al+15 V+ .Mn-l-fli 8 Mu+Ti+0.1 Pd 5 Mo+Ti Corrosion rate (mils/year)boiling H 04, 10%

' Dissolved In the tables above, the corrosion rate of titanium, withoutany addition, is given first to serve as a comparison. It will beobserved that all the noble metals tested were extremely effective inreducing the corrosion rate of both titanium and its alloys. It will beobserved that as little as'0.005 percent by weight palladium produces afourfold decrease in the corrosion rate of titanium in boiling 3.0percent HCl. It will further be observed that, in general, additions ofnoble metals reduce the rate of corrosion of titanium and its alloys atleast an order of magnitude, even in boiling non-oxidizing acids ofconcentrations as high as 55.0 percent.

The proportion of the noble metal addition may be varied according tothe anticipated use. Where contact with more rigorous environments isanticipated, such as strong concentrations of acid, more noble metal isrequired. FIGS. 1 and 2 demonstrate the eifectiveness of platinum andpalladium in decreasing the rate of corrosion of titanium inhydrochloric and sulfuric acid, respectively. Referring to FIG. 1,whereas the addition of 0.035 percent palladium is effective in reducingthe corrosion rate of titaniumrexposed to 10.0 percent boiling HCl tothe level of approximately 100 mils penetration per year, only 0.005percent is necessary to reduce the rate to the same level when titaniumis exposed to 3.0 percent boiling HCl. Referring to FIG. 2 whereas theaddition of 0.2 percent platinum is effective in reducing the corrosionrate of titanium exposed to 10 percent boiling H 80 to the level ofapproximately 50 mils penetration per year, only 0.015 percent isnecessary to reduce the rate to the same level when titanium is exposedto 1 percent boiling H 50 For best results, in conditions where exposureto strong solutions is encountered, the additive is used in amounts of0.1 to 0.5 percent. With amounts below 0.1 percent noble metal addition,the desired improvement in corrosion resistance may not be needs. Ascited previously, as little as.005 percent of noble metal addition totitanium may sufiice for low concentrations of acids. With amountssubstantially in excess of about 2.0 per- 4 cent, no marked degree ofimprovement results. The noble metals may be present in the alloy eithersingly or in any combination with each other.

The titanium-base alloys shown in Table II also exhibit improvedcorrosion resistance in non-oxidizing media when alloyed with one of thenoble metals listed. It is seen from Table II that alloys containing atleast about 80 percent by weight titanium are benefitted. Of especialsignificance is the titanium-molybdenum-noble metal alloy shown in TableII.

Some alloys of titanium and molybdenum are known to possess resistanceto corrosive non-oxidizing media because of the resistance to such medianaturally possessed by molybdenum. The resistance to non-oxidizing mediaof a titanium-40 percent molybdenum alloy is much greater than theresistance of pure titanium in such media. However, the increasedresistance to non-oxidizing media poossessed by this alloy is acquiredat the expense of the resistance to oxidizing media normally possessedby titanium. With amounts of less than 40 percent molybdenum intitanium, the resistance to non-oxidizing media is not as great. Below15 percent molybdenum, a titanium molybdenum alloy shows little of thisresistance to non-oxidizing media. However, by the addition totitanium-molybdenum alloys of one of the noble metals selected from thegroup ruthenium, rhodium, palladium, osmium, iridium, platinium, goldand rhenium, a corrosion resistance alloy is obtained that is suitablefor use in both oxidizing and non-oxidizing media. The titaniumbasealloy should contain from about 8 percent to 20 percent by weightmolybdenum, and may contain up to 5 percent by weight of one or more ofthe noble metals. A preferred range of composition is from 8 to 20percent molybdenum, from 0.05 to 2 percent palladium, and the balancetitanium and incidental impurities. Any noble metal from the grouplisted above may beused; but because of economic considerations, it ispreferred to use palladium.

For satisfactory commercial use :an alloy may be prepared that consistsof from 12 to 14 percent molybdenum, about 0.2 percent palladium, andthe balance titanium and incidental impurities. The alloys shown inTable V II include some high-molybdenum titanium-base alloys which showgreatly improved resistance to non-oxidizing media, but do not exhibitthe re'sistance'to oxidizing'media found in the alloys of thisinvention. For example, an alloy consisting of about 10 percentmolybdenum, about 0.2 percent palladium, and the balancetitanium'willfshow about the same corrosion resistance in boiling 5percent hydrochloricacid as titanium-20 percent molybdenum alloy;however, the low-molybdenum alloy of this invention has ten timesgreater resistance to an oxidizing environment, such as boiling percentnitric acid, than the 20 percent molybdenum alloy.

TABLE III Effect of Noble Metal Additions on Corrison Rate of ChromiumTable -III indicates the results of noble metal additions tosubstantially pure chromium. Table IV provides the same information asto noble metal additions to binary and tertiary chromium-base alloys.The percentages given in each case are percentages by weight of thetotal alloy.

TABLE IV Efiect of Noble Metal Additions on Corrosion Rate ofChromium-Base Alloys 1 Dissolved.

In Table III, the corrosion resistance of chromium without any additionis given first to serve as a comparison. Pure chromium, it will benoted, dissolves in both boiling sulfuric acid and boiling hydrochloricacid in the concentrations indicated. It will be observed that all thenoble metals tested were extremely effective in reducing the corrosionrate of chromium. The degree of efiFectiveness varies with the quantityof noble metal added. It will be observed that as little as 0.1% byweight platinum renders chromium practically insoluble in 20% boilingsulfuric acid. Although the corrosion rate appears to increase withincreasing amounts of platinum up to 5%, it will be understood that nosignificant difference actually exists between the various rates exceptfor the 0.5 osmium rate. The 0.5 osmium addition, while it decreases therate drastically, is not as elfective as an equal amount of platinum. Itwill further be observed that, in general, additions of noble metalreduce the rate of corrosion of chromium in boiling non-oxidizing acidsof concentrations as high as 60%. The proportion of noble metal additionmay be varied according to the anticipated use. Where contact with amore rigorous environment is anticipated, such as strong concentrationsof acid, more noble metal is required. For best results in conditionswhere exposure to strong solutions is encountered, the additive is usedin amounts of between 0.05 and 0.5%. With amounts below 0.05% noblemetal addition, the desired improvement in corrosion resistance may notbe suflicient to meet the particular needs. As little as 0.005% of noblemetal addition may suflice for low concentration of acid, however. Withamounts substantially in excess of about 5%, no marked degree ofimprovement results. The noble metals may be present either singly or incombination with each other.

In Table IV, the corrosion rate of several chromiumbase alloys, togetherwith the rate of the same alloy with the platinum addition, is given forcomparison. It will be seen that the base alloy readily dissolves inmoderate to strong concentration of either sulfuric or hydrochloricacid. Although a chromium base alloy containing 40 percent iron andpercent molybdenum is able to withstand the lower concentrations higherthan 20%. However, when only 0.5% platinum is added to the base alloy, avery marked decrease in the rate of corrosion results. The elfectivenessof the noble metal addition, as before, depends on the strength of thecorrosive medium and on the amount of noble metal added. Moreover, somealloys are more efliectively protected by the same amount of noble metaladdition than others. An 0.5% addition of platinum, for instance, whileit decreases the rate of corrosion of a chromium-base alloy containing 2percent Cu in 40 percent boiling sulfuric acid, is not as effective withthis alloy as with a chromium-base 10 percent molybdenum, or an alloypercent iron, 10 percent molybdenum chromium. Additions of noble metalsto chromium electrodeposit.

The alloys of the present invention may be prepared according to currentmetallurgical practice, although the invention is not limited by themethod of preparation. The ingredients may be in any commercially pureform since the invention is not limited to the degree of itsconstituents.

The description of the invention above has been in terms of its specificembodiments. Modifications and equivalents will be apparent to thoseskilled in the art and this disclosure is intended to be illustrativeof, but not necessarily to constitute a limitation upon, the scope ofthe invention.

This application is a continuation in part of my copending applicationSerial No. 732,793, filed May 5, 1958, now abandoned.

What is claimed is:

1. A corrosion-resistant alloy consisting essentially of from about0.005 to about 5 percent by weight in the aggregate of at least onemetal selected from the group consisting of ruthenium, rhodium,palladium, osmium, iridium, platinum, and rhenium, and the balancetitanium and incidental impurities.

2. A corrosion-resistant alloy consisting essentially of from about 0.05to about 2 percent by weight in the aggregate of at least one metalselected from the group consisting of ruthenium, rhodium, palladium,osmium, iridium, platinum, and rhenium, and the balance titanium andincidental impurities.

3. A corrosion-resistant alloy consisting essentially of about 0.05percent by weight palladium, and the balance titanium and incidentalimpurities.

4. A corrosion-resistant alloy consisting essentially of about 0.05percent by weight platinum, and the balance titanium and incidentalimpurities.

5. A corrosion resistant alloy consisting essentially of from about 8 toabout 20 percent by weight molybdenum, from about 0.05 to about 2percent by weight in the aggregate of at least one metal selected fromthe group consisting of ruthenium, rhodium, palladium, osmium, iridium,platinum, and rhenium, and the balance titanium and incidentalimpurities.

6. A corrosion-resistant alloy consisting essentially of from about 8 toabout 20 percent by weight molybdenum, from about 0.05 to about 2percent by weight palladium, and the balance titanium and incidentalimpurities.

7. A corrosion-resistant alloy consisting essentially of from about 12to 14 percent by weight molybdenum, about 0.2 palladium, and the balancetitanium and incidental impurities.

8. A corrosion resistant alloy consisting essentially of about 0.2percent by weight palladium, and the balance titanium and incidentalimpurities.

9. A corrosion-resistant alloy consisting essentially of from about0.005 to about 5 percent by weight in the aggregate of at least onemetal selected from the group consisting of ruthenium, rhodium,palladium, osmium, iriditun, platinum, and rhenium, and the balancesubstantially all a metallic material selected from the group consistingof titanium and alloys of titanium with at least one element selectedfrom the group consisting of aluminum, vanadium, manganese, molybdenum,and carbon containing :at least about percent by weight titanium.

10. A corrosion-resistant alloy consisting essentially of from about0.05 to about 2 percent by weight in the aggregate of at least one metalselected from the group consisting of ruthenium, rhodium, palladium,osmium, iridium, platinum, and rhenium, and the balance substantiallyall a metallic material selected from the group consisting of titaniumand alloys of titanium with at alloy containing consisting of 40 and thebalance may also be made 7 7 least one element selected from the groupconsisting of FOREIGN PATIENTS aluminum, vanadinm, manganese,molybdenum, and car- 18 212 Great Britain July 9, 1914 bon containing atleast about 80 per-gent by weight ti- M1913 +11; V tanium. 1

5 OTHER EEXEsR N E R n e Ci d in h fi eo i PM? Pietrokowsky et a1.:Journal of Metals, v01. 8, August UNITED STATES PATENTS 1956, pages930-935. Published by the A.1.M.E., New

7 York, N.Y. 2247755 Hansel et July 1941 ASM Metals Handbook, 1948edition, page 22, pub- 29659669 M NW1?! 1953 10 lished by the ASM,cleveilnnd, 01110.-

1. A CORROSION-RESISTANT ALLOY CONSISTING ESSENTIALY OF FROM ABOUT 0.005TO ABOUT 5 PERCENT BY WEIGHT IN THE AGGREGATE OF AT LEAST ONE METALSELECTED FROM THE GROUP CONSISTING OF RUTHENIUM, RHODIUM, PALLADIUM,OSMIUM, IRIDIUM, PLATINUM, AND RHENIUM, AND THE BALANCE TITANIUM ANDINCIDENTAL IMPURITIES.